1. Field of the Invention
This invention relates generally to modular implantable orthopedic prostheses, and particularly those which are adjustable in size to fit a given patient's needs.
2. Description of the Prior Art
Various prostheses have heretofore been designed to replace one or both components of a ball and socket hip joint. Generally the ball portion is connected to an arm composed of a neck and a stem or shaft which stem or shaft is embedded in the intramedullary canal of the proximal femur for hip reconstruction. Such prostheses are often formed with an integral stem and neck portion. Often a removable ball or head element is positioned on the proximal end of the neck. See, for example, U.S. Pat. Nos. 4,012,795 or 4,459,708.
Recently, the assembly of modular structures together from a number of replaceable parts available in a variety of sizes have been used. With such prostheses, it is possible to replace either the head portion or trochanteral portion of the prostheses, or both, without removal of the stem from the bone cavity during implantation. U.S. Pat. Nos. 4,608,055, 4,676,979 and 4,693,724 are all illustrative of such approaches. The latter patent also discloses the possibility that the angle at which the neck protrudes from the proximal end of the femur may be adjusted without removal of the stem by pivoting the neck on the end of the implanted stem.
These prior art devices, however, failed to provide a means for varying the angle between the axis of the trochanteral module and the axis of the stem so that the actual angulation (sometimes referred to as anteversion) or slope of the proximal end of the femur might be duplicated by adjustment of said angle. U.S. Pat. Nos. 5,002,581 and 5,201,882 to Paxson et al. filled such a need, by providing a modular device and instrumentation for implanting such device with the proper anteversion to match that of a patent's anatomy. The components of Paxson's device are secured together using complementary standard tapered connections (for example, a Morse taper may be used).
Other modular hip prostheses have been proposed, which are said to address various objects of design and use, among these the achievement of a "custom fit". For example, U.S. Pat. No. 4,995,883 to Demane et al. discusses using transitional sections of variable length between the several components of the device, secured together via combinations of a locking screw and tapered fittings. U.S. Pat. No. 5,002,578 to Luman discloses a modular hip having a neck inserted via a shouldered member to a unitary trochanteral/stem component, with a locking screw running through its shoulder into the trochanteral/stem component to secure the two components together. U.S. Pat. Nos. 5,080,685, 5,181,928, 5,286,260 and 5,370,706, all to Bolesky, each provide a modular prosthesis kit, capable of interoperative assembly by the surgeon, who chooses the proper size of components prior to implantation. U.S. Pat. No. 5,108,452 to Fallin shows a modular hip having extension sleeves to adjust the length between the ball and neck, as well as additional pads to increase the cross-sectional shape of the prosthesis body. U.S. Pat. No. 4,876,917 to Kranz et al., discloses a modular hip prosthesis having a stem with a distal tip that is radially expandable to anchor the stem against the medullary canal wall.
U.S. Pat. No. 4,846,839 to Noiles discloses a modular prosthesis design, alternatively adaptable to either total hip or knee arthroplasty, which presents a stepped contour interface with the patient's bone. The components of this design are connected via conventional tapers. A further type of device used for the fixation of modular prosthesis components is sold by H. D. Holmes under the registered trademark Spiralock.RTM., consisting of a clamping screw which fastens a standard taper connection together, e.g., connecting either the tibial tray or femoral component of a total knee joint to its respective stem. A further example of the use of such locking screws in a modular hip prosthesis is found in U.S. Pat. No. 5,397,360 to Cohen.
U.S. Pat. No. 5,405,398 to Buford, III, et al. discloses a knee prosthesis with a femoral component having a pin including a split ring which expands to keep the pin in place. U.S. Pat. Nos. 5,531,792 to Huene and 4,011,602 to Rybicki et al. each show bone fixation plugs having radially expanding members to apply compressive forces against the surrounding bone and promote in growth of the tissue into the member. Neither of these contemplate an improved mechanism for connecting the components of modular orthopedic implants of the type used in large or small total joint arthroplasty.
The modular knee and hip joint prostheses, described above, address the need for either or both the ball component or trochanteral module component to be removed if replacement becomes necessary without extraction of the stem from the bone canal. Different size balls or trochanteral components could also be substituted should the surgeon decide that such revision is necessary after a period of time. These conventional devices also contemplate selecting from a variety of sizes of their components, in order to match the anatomy of a given patient as closely as possible within the inherent variability of the assembly.
However, the modular systems, notwithstanding the variability offered in their assemblage of specifically sized components, fail to provide an infinite variability within a give size range while creating an assembly of enhanced biomechanical strength. That is, the prior assemblies introduce torsional tresses at the junctures of their components which do not necessarily reflect a unitary construction. Moreover, a wide array of sizes must be kept in stock during surgery to match a patient's anatomy.
Therefore, there is a need for a prosthesis which relies upon an enhanced means of connecting its components together, while further providing infinite adjustability within a given size range, while forming an assembly which biomechanically functions as an integral structure.